The present invention relates to a negative electrode for non-aqueous electrolyte rechargeable (secondary) batteries, and a production method for the same, of which the capacity is high and of which the discharge capacity has been improved so as not to deteriorate through the cycle usage.
In recent years, the demand for lithium ion rechargeable batteries, which have high electromotive force and high energy density, has greatly expanded along with the development of mobile communication equipment and portable electronic appliances. Lithium metal can be used for the negative electrode.material of this type of lithium rechargeable batteries in order to gain high energy density. However, dendrite deposits on the negative electrode at the time of charging, and when the charging and discharging are repeated the dendrite grows to stick out of the separator to reach the positive electrode side, which may cause the problem of internal short circuit.
Moreover, the deposited dendrite occupies a relatively large surface area, which enhances the reaction activation, and reacts with the solvent in the electrolyte on the surface so as to form an interface film like solid electrolyte, which lacks the electron conductivity. Thereby the internal resistance of the battery becomes high or a particle isolated from the network of the electron conductance are generated, which becomes the factor of lowering the charging and discharging efficiency, that is the cycle characteristics. In this way, the lithium rechargeable batteries where lithium metal is used as the negative electrode material have problems with the safety and the cycle characteristics.
At present, a battery has been put in practical use by utilizing a carbonaceous material, which can store and desorb a lithium ion as the negative electrode material alternating the lithium metal. Since no metal lithium deposits on the negative electrode made of carbonaceous material, usually there is no problem of the internal short circuit due to dendrite. In the case that graphite is used as the carbonaceous material, however, the theoretical capacity is 372 mAh/g, which is only one tenth of the theoretical capacity of lithium as a simple metal, and therefore, a capacity of a satisfactory level can not be obtained.
As a negative electrode active material, which can make the capacity higher, Sn of a simple metal is known which forms a compound with lithium. The compound of Sn which includes the largest amount of lithium is represented by the formula Li22Sn5, and in the case this compound is used, typically no metal lithium deposits and there is no problem of the internal short circuit due to dendrite. And the electrochemical capacity between this compound and Sn as a simple substance is 993 mAh/g, which is larger than the theoretical capacity of graphite.
In addition, a negative electrode material which comprises an intermetallic compound including at least one of the group 4B elements, P and Sb and having a crystal structure of CaF2 type, ZnS type or AlLiSi type has been proposed (the Japanese unexamined patent publication Hei-9(1997)-63651) as a compound negative electrode material of which the capacity is higher than that of the carbonaceous material. And a composite particle, which comprises an active material as a simple substance or a compound capable of the intercalation and the desorption of the lithium ion and carbonaceous material has been proposed as the negative electrode material (the Japanese unexamined patent publication Hei-11(1999)-135120).
However, there is a problem with Sn, which forms a compound with lithium and has a higher capacity than that of the above described carbonaceous material, in that Sn is inferior to the carbonaceous negative electrode material in the charging and discharging cycle characteristics. That is to say, because of the expansion caused by the formation of a compound with an stored lithium due to the charging reaction and contraction caused by a lithium desorption due to the discharging reaction, the active material particle becomes minute or fine. In other words, two phases with a volume difference are repeatedly converted to each other through the charging and discharging reaction, which causes the distortion of the material and generates a crack so as to make the active material particle further minute. Between the minute active material particles the electron conductivity is lost which leads to electrochemical inactivity, and the charging and discharging capacity is lowered.
On the other hand, a negative electrode material, which comprises an intermetallic compound containing at least one of the group 4B elements, P and Sb and has the crystal structure of CaF2 type, ZnS type or AlLiSi type, has been proposed as a negative electrode material having an improved cycle life characteristics e been improved instead of the above described simple substance such as Sn (the Japanese unexamined patent publication Hei-9(1997)-63651). This material is supposed to have better charging and discharging cycle characteristics than that of the Lixe2x80x94Pb alloy negative electrode material and have a higher capacity than that of the graphite negative electrode material. However, the discharging capacity in the charging and discharging cycles, up to 10 to 20 cycles, decreases remarkably and even the capacity of Mg2Sn, which seems to have the best characteristics, decreases to approximately 70% of the initial capacity after about 20 cycles. It seems that this is because even the active material particle in those materials expand or contract slightly, which makes it difficult to maintain the contact condition with the conductive agent after the charging and discharging, therefore, the contact degree between the active material particle and the conductive agent is deteriorated.
Moreover, a composite particle, which comprises an active material formed of a simple substance or a compound capable of the intercalation and desorption of the lithium ion and a carbonaceous material, has been proposed as a negative electrode material (the Japanese unexamined patent publication Hei-11(1999)-135120). The composite particle of this active material and the carbonaceous material is obtained by coating the active material comprising a simple substance or a compound capable of the intercalation and desorption of the lithium ion with the carbonaceous material. However, either the simple substance or the compound forming the internal active material stores or desorbs the lithium ion not like the carbonaceous material, which coats the surface of the active material, and therefore, the particle itself expands and contracts as described above.
In addition, it is difficult to coat only the surface of the active material including a Sn atom with the carbonaceous material and it is also considered difficult to prevent the active material from becoming deteriorated due to the expansion and the contraction.
It is an object of the present invention to provide a negative electrode material for non-aqueous electrolyte rechargeable batteries, which uses an active material particle including the element Sn of high capacity, and which maintains the contacts between the active material particle and the conductive agent without breaking into minute particle even after the expansion and the contraction are repeated along with the absorption and desorption of lithium in an electrochemical manner so as to improve the charging and discharging cycle life characteristics.
In order to solve the above described problems, the present invention provides a negative electrode for non-aqueous electrolyte rechargeable batteries comprising; an active material composed of a lithium ion storing phase, which contains at least Sn and a lithium ion non-storing phase, and an electronically conductive material, which coats at least a part of the surface of said active material. In other words, the present invention provides a negative electrode for non-aqueous electrolyte rechargeable batteries comprising an active material in the form of a composite particle of; an active material particle having a lithium ion storing phase, which contains at least Sn as the component and a lithium ion non-storing phase; and an electronically conductive material which coats at least a part of the surface of said active material particle.
In the negative electrode for non-aqueous electrolyte rechargeable batteries, it is preferable that the lithium ion storing phase further contains, as another component, at least one type of element selected from the group consisting of the group 2 elements, the transitional elements, the group 12 elements, the group 13 elements and the group 14 elements in the periodic table, and the another component and Sn form a solid solution or an intermetallic compound.
Further, it is preferable that the electronically conductive material coats 10% or more of the entire surface of said active material.
Further, it is preferable that the electronically conductive material is at least one selected from the group consisting of an electronically conductive polymer, a carbonaceous material and a metallic material.
Further, it is preferable that the carbonaceous material includes a fibrous carbon.
Further, it is preferable that the fibrous carbon is in a fiber shape having a lattice spacing d002 of (002) plane of 3.35 to 3.70 xc3x85, a length of 1 to 20 xcexcm and a diameter of 0.1 to 0.5 xcexcm, and the fibrous carbon in the carbonaceous material is in a range of 1 to 20 wt % of the active material and the electronically conductive material.
Further, the present invention provides a production method of an active material for a negative electrode for non-aqueous electrolyte rechargeable batteries, said active material being composed of a lithium ion storing phase, which contains at least Sn, and a lithium ion non-storing phase; and an electronically conductive material, which coats at least a part of the surface of said active material. In other words, the present invention provides a production method of a composite particle for a negative electrode for non-aqueous electrolyte rechargeable batteries, said composite particle comprising an active material particle having a lithium ion storing phase, which contains at least Sn as the component, and a lithium ion non-storing phase; and an electronically conductive material, which coats at least a part of the surface of said active material particle.
The production method comprises the steps immersing a negative electrode active material particle into an organic solvent containing a conductive polymer material dissolved, and drying said active material particle to coat at least a part of the surface of said active material particle with said conductive polymer material to give said active material.
Another production method of the present invention comprises the step of applying a mechanical energy comprising a compressive force and a grinding force to a mixture of a negative electrode active material particle and an electronically conductive material to coat at least a part of the surface of said active material particle with said electronically conductive material through a mechanochemical reaction to give said active material.
Further, the present invention provides a non-aqueous electrolyte rechargeable battery, comprising a positive electrode capable of a reversible electrochemical reaction of a lithium ion, a non-aqueous, electrolyte comprising a lithium salt dissolved in an organic solvent and the above-mentioned negative electrode.
In particular, the above described active material, i.e. composite particle, which functions as a so-called negative electrode material, comprises a core active material particle and a shell electronically conductive material coating, in which the active material particle enhances the capacity and the conductive material maintains the contact with the active material particle even when the active particle repeats the expansion and contraction along with the storing (absorbing) and disorption of lithium ion in an electrochemical manner and, therefore, the cycle characteristics can be maintained and improved.